Uranium contamination in soil and water is of global concern and has been identified at a number of sites worldwide. Contamination may occur as a result of a variety of different activities, both natural and anthropogenic, including military testing, radiation accidents, nuclear fuel cycle activities (uranium mining, ore processing, fuel fabrication and reprocessing), electricity generation, mining and processing of other natural resources, and application of radionuclides in other industries.
In oxygen-containing groundwater, uranium is generally found in the hexavalent oxidation state. In waste, uranium is present primarily as soluble salts of the uranyl ion (UO22+). The oxidized or hexavalent (VI) state of uranium is highly soluble and mobile, while the reduced or tetravalent (IV) state is relatively insoluble and, thus, immobile. As U(VI) is transported through groundwater, it can bond to minerals or carbonate and calcium species commonly found in groundwater. The latter scenario is problematic because the U(VI) remains highly mobile.
When reduced from the oxidation state, U(VI), to a lower oxidation state, such as U(IV), the solubility of uranium decreases and it becomes immobilized. In contrast to U(VI), U(IV) does not form soluble solids even in the presence of calcium and carbonate.
As U(VI) is transported through groundwater, it can bond to surfaces of minerals, a process which may retard its transport. It has recently been shown, however, that U(VI) also bonds strongly to the common groundwater species carbonate and calcium to form stable dissolved ternary complexes, which can effectively compete with mineral surfaces as “reservoirs” for U(VI). As a consequence, significant amounts of U(VI) remain in groundwater, thus maintaining relatively high mobility for U(VI), a highly undesirable scenario. Conversely, the tetravalent oxidation state, U(IV), forms sparingly soluble solids, even in the presence of dissolved carbonate and calcium, and thus tends to be relatively immobile.
Various strategies for remediation of uranium from groundwater and soil have been proposed in order to reduce the detrimental effects of uranium contamination on ecosystems and local communities. These methods are sometimes able to reduce uranium concentrations below regulatory limits [the U.S. EPA Maximum Contaminant Level (MCL) for U is 0.13 μM]. These strategies include physical, chemical and biological technologies. For example, iron barriers, soluble reductive agents, microbial stabilization via reduction and precipitation, and emplacement of solid phosphate barriers have been pursued as potential technologies to remediate uranium from a contaminated environment.
Currently, one of the most researched methods of uranium remediation is microbial mediated reduction of soluble uranyl species. This technique typically relies on injection of organic carbon into the contaminated environment to stimulate microbial U(VI) reduction to U(IV) solids. Under reducing conditions, microbial bioreduction produces elevated concentrations of bicarbonate and organic ligands from microbial utilization of organic carbon which promotes higher aqueous U(VI) concentrations. Consequently, organic carbon concentrations must be kept at concentrations high enough to maintain reducing conditions, but low enough to limit the formation of aqueous U(VI) carbonates. In addition, reducing conditions in the contaminated environment must be maintained due to the fact that U dissolves upon a return to the original oxidizing conditions of the subsurface environment. Another proposed remediation method is precipitation of uranium with phosphate in contaminated sediments. Phosphate reacts with U(VI) to form aqueous and ternary surface U(VI) complexes, poorly soluble uranyl phosphate precipitates, and U(VI) adsorbing phosphate minerals.